The present invention relates to a method of forming through-holes in printed wiring board substrates; and more particularly to the formulation of micro through-holes in the same accurately and effectively by means of a chemical etching process.
Conventional methods of forming through-holes in printed wiring boards employ mechanical drilling, laser drilling and plasma etching among others.
Regarding mechanical drilling, it is very difficult to form micro through-holes of 100 microns or less due to the difficulty in producing drills of that size that do not readily break. Also, even using a drill size of 250 microns it takes 4-6 hours to drill 10,000 holes, an inordinate amount of time. Further, mechanical drilling creates burrs, dust and debris that often block the holes or adhere to the surface of the substrate material requiring additional cleaning process. If the substrate is clad with a metal, the metal is smeared into the hole. And, the drilling apparatus required is very expensive and additional expendables such as entry and back-up boards also add to cost.
Laser drilling and plasma drilling processes are incapable of forming holes in substrate materials containing glass fibers which disallows use of the most widely used and inexpensive substrates which normally do contain glass fibers. In addition, equipment to perform laser or plasma drill is also very costly.
The foregoing discussion of the prior art is taken largely from U.S. Pat. No. 5,352,325 to Kato who proposes forming micro through-holes in circuit boards by selectively dissolving or etching using alkaline potassium permanganate in the case of a circuit board material not containing glass fibers, and using fluoride chemicals in the case of a board material containing glass fibers. According to Kato, a printed wiring board substrate, consisting of a insulative layer (usually composed of resinous material with or without glass fiber reinforcement) covered on both sides with a conductive layer, is covered with an etching resist except for specified locations where holes are to be formed; a selected chemical etchant is brought into contact with said surfaces and selectively dissolves the exposed parts of the conductor layer; the previously applied etching resist is removed; the conductor surface is then exposed to a strong alkaline solution and a solvent which softens the exposed areas of the insulative core material where holes are to be opened; the surface is then brought into contact with either alkaline potassium permanganate, concentrated sulfuric acid, chromate or similar solution, under agitation whereby to selectively remove by chemical dissolution the exposed resin core material; this treatment is interrupted by acidic washing followed by a neutralizing solution wash. The substrate is then exposed to a fluoride-containing solution under agitation which dissolves the glass fibers, thus completing the etching process and forming the micro through-holes. Finally, the through-holes are plated to provide electrical interconnection between the conductive layers on both sides of the board.
Using the aforedescribed method, Kato reports that it is possible to precisely determine the exact size, shape and location of through-holes. While the method disclosed by Kato may achieve certain of his stated advantages, it is believed that the Kato method has not achieved any degree of commercial success due to its complexity and the need to employ multiple solvents and solvent application steps which dramatically increases manufacturing time and cost. Moreover, the use of multiple solvents and solvent application steps increases disposal costs, i.e. of spent solvents.